1. Field of the Invention
The present invention relates to an energy storage device, and more particularly to an energy storage device in which an electrolyte permeability index of a separator is greater than an electrolyte permeability index of electrodes, and in which unit fibers in the separator are arranged irregularly.
2. Description of the Related Art
Generally) batteries and capacitors are representative electric energy storage devices.
Ultra capacitor, also called super capacitor, is an energy storage device whose features are in the middle between electrolytic condensers and secondary batteries. Due to high efficiency and semi-permanent life span, the ultra capacitor is considered as a next-generation energy storage device that is usable in parallel with or instead of secondary batteries.
Ultra capacitors may be classified into EDLC (Electric Double Layer Capacitor) and pseudo capacitor, depending on energy storing mechanism.
Pseudo capacitor utilizes accumulation of charges on or near an electrode surface, while EDLC utilizes absorption of charges on an electrical double layer at the interface between an electrode and an electrolyte.
The electrical double layer of EDLC is formed at a contact surface between an electrode material surface and an electrolyte by using a material with side surface area such as activated carbon as an active material of the electrode.
In other words, charge layers with different polarities are formed at a border surface of an electrode and an electrolyte solution due to the electrostatic effect. The charge distribution formed as above is called an electrical double layer, and this phenomenon gives a charging capacity like batteries.
However, charging/discharging characteristics of the electrical double layer capacity are different from those of batteries. A voltage-to-time feature of a general battery is exhibited as a plateau graph during a charging/discharging process, but a voltage-to-time feature of an electrical double layer capacitor is exhibited as a linear graph during a charging/discharging process.
Thus, an amount of charged/discharged energy of the electrical double layer capacitor may be easily calculated by measuring its voltage.
Meanwhile, seeing the electricity storing mechanism in the above electrical double layer capacitor, charges are stored in the electrical double layer formed at the interface of an electrolyte, so the electrical double layer capacity utilizes physical accumulation of charges for electricity storage, differently from batteries utilizing chemical reaction. Thus, the electrical double layer capacitor exhibits no performance deterioration caused by repeated uses, and ensures excellent reversible characteristics and long life span.
Thus, the electrical double layer capacitor does not need maintenance, and it is sometimes used instead of batteries in applications demanding long life span.
Meanwhile, the electrical double layer capacitor as mentioned above uses the principle of intercalating/disintercalating charges to/from an electrical double layer formed at the interface between an electrode and an electrolyte, so it has fast charging/discharging characteristics. Accordingly, the electrical double layer capacitor is very suitable as main or auxiliary power source of electric vehicles, road sign lamps and UPS (Uninterrupted Power Supply), which demand large capacities, as well as auxiliary power source of mobile communication devices such as cellular phones, notebooks and PDA.
An electrode of the electrical double layer capacitor having various usages as mentioned above should ensure high energy through wide specific surface, high output through low specific resistance, and electrochemical stability through electrochemical reaction control at the interface.
Thus, activated carbon powder or activated carbon fiber with wide specific surface area is most frequently used as a main material of the electrode, and conductive material or metal powder is mixed or applied thereto by means of injection coating to realize low specific resistance.
In addition, more stable electrode materials are now studied and tinder development by controlling electrochemical side reactions occurring at the electrode interface in various ways.